What characterizes a dormer window?

A dormer window is specifically defined as a vertical window that is set within a structural extension of the roof, known as a dormer. This feature allows for natural light and ventilation into the upper spaces of a building, typically in an attic or loft area. The vertical orientation of the window creates a distinctive appearance, often complementing the slope of the roof.

The other options illustrate different window types or architectural features that do not accurately describe a dormer. A horizontal band of glass refers to a type of window configuration commonly found in modern designs but does not fit the dormer definition. A round window above a door describes a different architectural element, often associated with classical designs, but also does not represent a dormer. Lastly, a window projecting from the wall typically refers to a bay or oriel window, which protrudes from a flat facade rather than being integrated into the structure of the roof like a dormer window. Thus, the defining characteristics of a dormer window are encapsulated in the choice that describes its vertical orientation and integration with the roof structure.